Unexpected formation of Fe^IIFe^IIIFe^II- and Fe^IIFe^IIFe^II-trinuclear hexapyridineoximate helicates: synchrotron single-crystal X-ray structures

Template condensation of 2-acetylpyridineoxime with butyl ester of 1,3-propanediboronic acid in the presence of iron(II) affords a poorly separable mixture of complex products. Similar results were observed when Fe3+ ions were used for cross-linking in place of the diboronic acid. Prolonged crystallization of these mixtures gave crystals of perchlorate salts of a (FeFeFeII)-Fe-II-Fe-III-trinuclear hexapyridineoximate cation and of an analogous (FeFeFeII)-Fe-II-Fe-II-intracomplex that crystallize with chloroform and dichloromethane solvent molecules, respectively. Both complexes were characterized by synchrotron single-crystal X-ray diffraction (XRD) experiments. The Fe-N distances of the two 'terminal' iron cations are virtually the same between these trinuclear complexes and unambiguously support assignments of oxidation number +2 and low-spin state (S = 0). The centers of the FeN6-coordination polyhedra adopt geometries resembling trigonal antiprisms with distortion angles phi of approximately 40 degrees. The FeO6-coordination polyhedra possess an almost ideal O-h geometry with phi close to 60 degrees. Each of these iron(II) or iron(III) mu(3)-metallocenters acts as a bifunctional Lewis acid that cross-links two pseudoclathrochelate entities, formed by the 'terminal' iron(II) ions, to give an almost linear (the corresponding intramolecular Fe(2)center dot center dot center dot Fe(1)center dot center dot center dot Fe(3) angles are close to 180 degrees) helical cation or intracomplex.